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1 ENGN4017 - Engineering DesignEEG Monitoring SystemThe EEG Porto-Cap™WelcomeThis presentation will discuss the progress made in the development of an EEG monitoring system by Engineering 4017 Project Group 5.My name is Anthony Collis and I will be presenting the first half of this morning’s session and Torrey Bievenour will be presenting the second half of the session.Team 5ENGN Engineering DesignAutumn 1999

2 Design Process Examine project domain Meet with intended customerMarchExamine project domainEEG measurement systemsMeet with intended customerDr Richard Clark, Flinders University SAIdentifying problems with existing skull cap systemRequirements for new system were formulatedDevelop conceptual solutions to problemsBrainstormingProject DecompositionSub-system groups focussing on specific problemsIntegrationSystem integrationPresentation, Final ReportThe Design ProcessTo start off, I will briefly go through the processes that Group 5 have undertaken to complete this project.The design of an EEG skull cap is definitely a systems engineering project. Engineering skills used includedMechanical EngineeringElectrical EngineeringTelecommunuications EngineeringChemistryProject ManagementCommunication, Negotiation and PresentationFinancial AnalysisAfter being set the project in March of his year, we performed a survey of existing work in the field of EEG skull caps so that we could anticipate customer requirements at the first meeting with the customer.We met with the Customer, Dr Richard Clark of Flinders University and gained insight into what he considered to be deficiencies in his own existing skull cap systems. From these sessions we formulated a set of requirements which became the focus of our project.Following a typical design path, we brainstormed for ideas and also researched innovative solutions that may already be in existance.From this process we divided into subgroups to complete the tasks, making it easier from a management point of view.When the subgroups had essentially completed their work we integrated the solutions and prepared the final report and presentation to show our progress to date.June

3 What is EEG? EEG = ElectroencephalogramMeasurement of brain wave patterns in hopes of understanding the mysteries of the mind.EEG is a system of measuring brain wave patterns and recording them in a format that allows interpretation by specialists in the field.The simplest EEG system illustrated above is made up of three parts - the electrodes which interface with the skull, an amplifier which amplifies the signals to an acceptable level, and a recording system for storing the measured signals.The typical magnitude of an EEG signal is in the range of +/- 200microvolts. This level of signal is very susceptible to interference from stray electromagnetic fields, therefore shielding and amplifying systems of high quality must be used to ensure accurate representation.Our group focussed on the medical applications of the EEG system as we felt that this was an achievable outcome in the short time frame available.

4 Problem StatementThe reliable measurement, over extended periods, of scalp EEG on consecutive occasions, during mental functions in everyday environments.After meeting with our customer, the following problem or mission statement was set. Some of the key phrases are:reliable measurement - implying that the system must be able to faithfully produce EEG signals at least as well as existing systemsconsecutive occasions - when we want to undertake studies on the same subject or patient we require them to be repeatable (this is standard scientific practice)everyday environments - the existing system does not allow operation in everyday environments. The system is bulky and requires extensive electromagnetic shielding in the form of a faraday cage. This is very much a lab setting not an everyday environment.From this problem statement a comprehensive set of requirements were developed. We will be discussing these key requirements and how well we satisfied them throughout the presentation.

5 Key Problem AreasIssues that needed to be addressed in any new system developed by the group:Lack of repeatabilityDifferent Head SizesLong setup timesDifficulty with lengthy experimentsUse of conductive gelsScalp abrasionNoise in signalLack of portability/environmental constraintsOur initial sessions gave us the problem statement but this statement is very broad. We identified the following as being the key problem areas for the project and we ambitiously set out to come up with an innovation that would solve the customer requirements in each of these areas.Lack of repeatability - the existing system made it very difficult to position the electrodes in the same place for each experiment.Different Head Sizes - the skull cap should be adaptable to the broadest range of head sizesLong setup times - To allow for everyday use and for the benefit of researchers and subject patients, setup times should be minimisedDifficulty with lengthy experiments - need to ensure that signal processing equipment and battery systems can handle the length of the experimentsUse of conductive gels - this adds substantially to the setup time and difficulty in operationScalp abrasion - limits the extent of repeatable experiments due to potential damage to the scalpNoise in signal - must be manageable if the system is to be used outside of a faraday cageLack of Portability - prevents operation in everyday environmentsWe have made inroads into all of these key problem areas through the project.In finding a solution, we have taken an ambitious approach to solving these problems in a medical research application

6 Problem DecompositionIt became apparent from the initial identification of requirements that a number of challenges were logically related to each other. The group focussed on three broad areasMechanical Design - the physical helmet design, positioning of the electrodes, accurate and repeatable positioning of the electrodes.Electrode design and selection - also addressing the issues of hygeine and preventing scalp irritation.Signal processing and transmitting signals back from the cap back to the base system.To address these challenges, a number of innovative design solutions have been incorporated in the skull cap.

7 M e c h a n i c a l Key RequirementsThese are the key requirements of the mechanical design for the skull cap.We set as a benchmark for best practice in electrode positioning the use of the 10/20 system of electrode positioning. Our cap design incorporates electrodes placed at the 10/20 system locations, but has flexibility to have electrodes positioned a other locations.The accuracy of the cap determines whether the experiments conducted are repeatable to any extent. We needed to assure that the electrodes could be positioned to within 1mm of where they were on the previous occasion. Our design satisfies this criteria by having three biometric reference points from which we can accurately align the helmet.The head size range of the cap should be broad enough to accommodate the majority of head sizes. The cap we have designed is flexible through the range 53 to 66 cm by the use of adjustable straps.The setup time should be minimal for both first use and subsequent uses. Our cap addresses these issues by having cliplock electrodes allowing them to be fitted after cleaning with ease, snaplock head size adjustment and retractable electrodes which can be threaded in and out to suit the requirements, in total allowing for the cap to be fitted in under 15 minutes.

8 M e c h a n i c a lCap DesignThe skull cap design is a rigid cap that had ventilation sections cut out to improve comfort for the subject.The helmet is adaptable to a wide variety of head sizes because of the flexible strap at the rear of the helmet which offers a range of head sizes from 51cm to 68cm head circumference.Each electrode is mounted on the helmet surface and is adjustable through a screw mechanism to adjust for different positioning and head sizes. Electrodes can be relocated to other points with ease.These features are apparent from the mechanical prototype of the helmet that was developed by the group.[Show model]Point outThis prototype is a simple model that simulates the positioning of the helmet. Due to cost limitations it was not possible to use real electrodes or circuitry to illustrate the design solutions of the electrode or signal processing groups.We created the prototype to verify placement and weight considerations as well as verifying that the setup time goal could be met.

9 Nathan in an early model.M e c h a n i c a lPositioningNasion/Ear reference pointsAdjustable straps for different head dimensions (53cm - 66cm)Electrodes retractable to suitNathan in an early model.Current model.One of the key requirements of the cap is that the cap must be positioned accurately for several experiments on different occasions.We achieve positioning accuracy by:Using a nasion interface unit which fits each subjects nasion. This prevents the helmet from tipping forward or backward.Using interfaces which measure the distance to the top of the ear.Measuring and locking down the head circumference measurement.Adjusting the protrusion of each electrode to suit the head circumference this is done by threading the electrodes in and out on a screw thread, as featured in the model.

10 Demonstration of Skull Cap Setup . . .M e c h a n i c a lDemonstration of Skull Cap Setup . . .1. Measure the head circumference and adjust the lock mechanism at the rear of the helmet to suit2. Ensure that all the electrodes are retracted so that no injury occurs while fitting.3. Place the helmet on the subject’s head4. Seat so that the reference points are correctly positioned, ears and nasion at the front. This fixes the helmet in three dimensional space.5. Thread in electrodes until eeg signals commence or good contact made.

11 E l e c t r o d e Key RequirementsThe electrode group’s focus was on the electrodes and issues relating to their maintenance and interaction with the subject user.A key requirement developed was that skull impedance must be reduced for the faithful reproduction of EEG signals. Using special purpose circuitry, this was achieved, and also ensured that conducting gels were not necessary.Irritation needed to be minimsed to prevent paitient discomfort. Our design causes little discomfort because the weight of the cap is transferred via straps, rather than the electrodes supporting the weight. Not having access to the electrodes themselves prevented us from achievingElectrode polarisation occurs during long EEG recording sessions where the flow of current causes electrolysis between the electrode and the moisture on the scalp. By choosing a specific electrode material which does not experience electrolysis, this requirement has been addressed, however once again due to the lack of availability of the electrodes it could not be tested. The literature in the field indicates that it will be.Cleaning issues were addressed by preparing a manual which stipulates the guidelines for cleaning and tests to ensure compliance.Scalp preparation - by the choice of electrodes and special purpose impedance reducing circuitry no preparation, abrasion or conductive gels are required.

12 E l e c t r o d e Electrode Selection Silver/silver chloride selectedMinimal polarisation characteristicDry electrode used (no gel required)Uses local impedance converting amplifierPellet electrode, 2mm diameterSmall electrode size ensures good hair penetration and contact with scalpThe electrode material chosen was silver-silver chloride for its minimal polarisation characteristics, allowing prolonged use in the field.We conducted extensive research and all literature that was found recommended these electrodes highly, confirming our design choice.A special purpose amplifier manufactured by Maxim corporation of the US is used to reduce the electrode/scalp impedance. Research papers in the Journal of Electrophysiology recommend this approach and it produces accurate representation of the EEG of a subject.This needs to be confirmed by our own research but the high cost of both the electrodes and associated circuitry prevented us from doing practical trials of the system. Therefore no gel or abrasion of the scalp is required.We chose to use a small diameter electrode as this allows greater positional accuracy and also ensures that the electrode can penetrate the hair to gain access to the scalp surface. Any hair will be brushed aside by the rounded tip of the electrode.2mm

25 Why you should upgrade to the Porto-Cap™ . . .Wireless communication – no huge cables to lug around, you are free to walk around the house just like with a cordless phone.Data recorded from each different electrode is clocked at exactly the same instant – at last, a device which accurately maps which bits of your brain that weren’t working during that embarrassing moment.No scalp preparation required – the feeling of blunt needles scraping your scalp will only be remembered in your nightmares.Porto-Cap™ uses dry electrodes only – that gooey conducting paste can now be used as axle grease.Zero signal drift – the amplifier and filter package has been hand picked so that the only interruptions during a testing period will involve rubble entering your building.Eight selectable sampling frequencies – if flexible data rates is you, then you won’t die hungry when wearing the Porto-Cap™.

26 ConclusionThe EEG Porto-Cap™ successfully enables repeatable and reliable measurements of EEG signals. It achieves this with little discomfort, no paste or gel and in conventional environments.

27 ENGN4017 - Engineering DesignThank you for coming, we would like to take this opportunity to answer any questions you may have.1. Measure the head circumference and adjust the lock mechanism at the rear of the helmet to suit2. Ensure that all the electrodes are retracted so that no injury occurs while fitting.3. Place the helmet on the subject’s head4. Seat so that the reference points are correctly positioned, ears and nasion at the front. This fixes the helmet in three dimensional space.5. Thread in electrodes until eeg signals commence or good contact made.Team 5ENGN Engineering DesignAutumn 1999